Heretofore, radiation images such as X-ray images have widely been employed in hospitals and clinics for the state of a disease. Specifically, over a long period of history, radiation images formed via intensifying screen-film systems have resulted in high photographic speed and high image quality, whereby even now, they are employed in hospitals and clinics in the world as imaging systems which simultaneously exhibit high reliability and cost performance.
However, types of the above image information are those of so-called analogue image information, and enable to achieve neither free image processing nor instantaneous electric transmission, which is realized in digital image information which has been developed in recent years.
Further, in recent years, digital system radiation image detection device, represented by computed radiography (CR) and flat-panel type radiation detectors (FPD) have appeared. These enable direct formation of digital radiation images and direct display images on image display devices such as a cathode tube or a liquid crystal panel can be achieved. When applying these radiographies, images are not always required to be formed on photographic film. As a result, the above digital system X-ray image detectors have decreased the need of image formation via silver halide photographic systems and have significantly enhanced convenience of diagnostic operation in hospitals and clinics.
As one of the digital technologies of X-ray images, computed radiography (CR) is presently employed in medical settings. However, sharpness is insufficient and spatial resolution is also insufficient, whereby its image quality level has not reached that of the screen-film systems. Further developed as a new digital X-ray image technology are flat-panel X-ray detectors (FPD) employing thin-film transistors (TFT), which are described, for example, on page 24 of John Rawland's report, “Amorphous Semiconductor Usher in Digital X-ray Imaging”, Physics Today, November 1997 and on page 2 of L. E. Antonku's report, “Development of a High Resolution, Active Matrix, Flat-panel Imager with Enhanced Fill Factor” of the magazine of SPIE, Volume 32, 1997.
In order to convert radiation to visible light, employed are scintillator panels which are prepared employing X-ray phosphors exhibiting characteristics of emitting light via radiation. However, in order to enhance the SN ratio during imaging at low dosages, it becomes necessary to employ scintillator panel at a high light emitting efficiency. Generally, the light emitting efficiency of scintillator panels is determined by the thickness of the phosphor layer and the X-ray absorption coefficient, while as the thickness of the phosphor layer increases, scattering within the phosphor layer of emitted light occur, which lowers sharpness. Consequently, when required sharpness for image quality is determined, the layer thickness is determined.
Of the above phosphors, cesium iodide (CsI) exhibits a relatively high conversion ratio from X-rays to visible light and it is possible that phosphors are easily formed in a columnar crystal structure via vapor deposition. Consequently, scattering of emitted light in crystals is retarded via optical guide effects, whereby it has been possible to increase the thickness of the phosphor layer.
However, when only CsI is employed, the light emission efficiency is relatively low. Therefore, a mixture of CsI and sodium iodide (NaI) at any appropriate mol ratio is deposited on a substrate in the form of sodium-activated cesium iodide (CsI:Na), employing vapor deposition, and recently a mixture of CsI and thallium iodide (TlI) at any appropriate mol ratio is deposited on a substrate in the form of thallium-activated cesium iodide, employing vapor deposition. The resulting deposition is subjected to a thermal treatment at temperature of 200° C.-500° C. as a post-process to enhance the visible light conversion efficiency, whereby resulting materials are employed as an X-ray phosphor. (refer, for example, Patent Document 2)
However, as an activator has different crystal structure from cesium iodide, higher concentration of the activator deteriorates sharpness due to distortion of the columnar crystal structure. Thereby, when unevenness of the activator concentration increases, problems of unevenness occur not only in sensitivity but also in sharpness.
In Patent Document 3, in order to increase columnar crystallinity of phosphor layer, disclosed is a method for manufacturing of radiation image conversion panel in which a phosphor layer is formed comprising steps of forming a columnar crystal structure of base phosphor material by a vapor deposition method, then building up a columnar crystal structure of the phosphor on the columnar crystal structure (growing a columnar crystal of the phosphor on the columnar crystal of base phosphor material in one-to-one correspondence). However, this invention substantially relates to a stimulable phosphor, the columnar crystal structure of base phosphor material fuses with a columnar crystal structure of the phosphor in places of the obtained phosphor layer, resulting in rather worse unevenness of sharpness.